(Dec. 8, 2008) -- Men may not be getting as much vitamin D as they did in the past, and that's not good, a new study says.

The reasons for the decline, however, aren't crystal clear, according to study author Anne C. Looker, PhD, of the National Center for Health Statistics, which is part of the CDC.

Apparently, men are drinking less milk these days and take more steps to protect themselves from the sun than they did in the late 1980s and early '90s, according to the study, published in the December issue of the American Journal of Clinical Nutrition.

In an email to WebMD, Looker writes that men's vitamin D levels have declined between 7% and 12% over the past 15 to 20 years.

She attributes most of the decline to changes in body mass index, decreases in milk intake, and increases in sun protection.

"We do not have data on reasons why men consumed less milk, unfortunately," she tells WebMD.

Looker and fellow researchers used data from National Health and Nutrition Examination Surveys to compare serum vitamin D levels in 18,000 participants in 1988-1994 with those of 21,000 in 2000-2004.

She tells WebMD that most of the difference in serum vitamin D levels was a result of "subtle changes in the laboratory assay method, which underscores the importance of standardizing the assay. Once the method difference was accounted for, there appeared to be a small (about 11 percent) decline in serum vitamin D levels of men between surveys.

"Values in women were not significantly different between surveys," she writes. "It appeared that combined changes in body mass index, milk intake, and sun protection behavior may have contributed to the decline in serum vitamin D between surveys."

The study says body fat is inversely related to serum vitamin D levels. But Looker and colleagues also say it's not known whether the much-publicized increase in the prevalence of overweight Americans has been accompanied by a decline in vitamin D levels.

Current recommended intake of vitamin D may be too low, according to researcher Anthony Norman, PhD, of the University of California-Riverside.

He expresses concerns about the adequacy of current daily intake recommendations important for good health.

Norman, who wrote an accompanying editorial, points out that the current intake recommendation, made in 1997, may be too low and too focused on vitamin D's actions on calcium and bone issues.

"The evidence that vitamin D is involved in a plethora of physiologic systems provides strong justification to re-evaluate optimal daily vitamin D intakes," he says in a news release.

The decline in vitamin D levels in men "is potentially disturbing if it represents the beginning of a downward trend," Norman writes in his editorial. "It will therefore be essential to repeat this study after another decade."

He points out that other studies have shown that people with lower serum concentrations had a higher incidence of cancer, "which reinforces the need to improve vitamin D nutritional status."